The present invention relates to apparatuses and to methods for heating motor vehicle seats.
More particularly, the invention relates to apparatus for heating a motor vehicle seat, the apparatus comprising:
a heater mat comprising an electrical resistance element connected to two terminals of an electrical power supply, advantageously a positive terminal and a negative terminal;
a measurement mat, e.g. for characterizing the occupant of the seat in terms of weight and/or position, said measurement mat comprising at least one capacitive sensor itself comprising at least two electrodes; and
control means for disconnecting the power supply to the heater mat and adapted to disconnect the electrical resistance element from at least one of the two terminals of the electrical power supply.
International patent application WO 02/06083 describes an example of one such heater apparatus, in which the power supply to the heating mat is interrupted immediately before picking up a measurement signal from a sensor.
However, it has been found that the measurements provided by that type of apparatus lack accuracy and reliability, with uncertainty concerning measurements of weight and of position then possibly being greater than 65%.
A particular object of the present invention is to mitigate that drawback.
To this end, according to the invention, apparatus of the kind in question further comprises:
switch means connected to at least two of the electrodes and adapted, in a first state, to charge the sensor electrically by imposing a charging potential to at least one of the electrodes, while taking another electrode to a reference potential, and in a second state, to discharge the sensor into an electric charge measurement circuit; and
the means for disconnecting the power supply to the heater mat disconnect the electrical resistance element from at least one of the two terminals of the electrical power supply while the switch means of the measurement mat are in the first and second states.
By means of these dispositions, the heating mat is out of operation throughout the time that each sensor is in operation. The shielding caused by the heating mat is thus reduced not only during the first step corresponding to a measurement-taking stage, but also during the second step corresponding to a stage of delivering measured magnitudes, contrary to the above-specified prior art apparatus which avoids the shielding effect only during the second step, whereas the shielding effect also disturbs measurement during the measurement-taking stage.
The shielding effect is further decreased when:
the control means for disconnecting the power supply to the heater mat are adapted to disconnect both terminals of the electrical power supply from the electrical resistance element while the switch means of the measurement mat are in the first and second states; and/or when:
the control means for disconnecting the power supply to the heater mat are adapted to raise at least one of the two terminals of the electrical power supply for the electrical resistance element to the charging potential while the switch means of the measurement mat are in the first state; it is thus possible to have either both electrical power supply terminals connected to the charging potential, or one of the two terminals connected to the charging potential while the other is floating; in the first case uncertainty on the measurement performed by each sensor can be less than 10%.
In another aspect, the invention provides a method of heating a motor vehicle seat, in which a heater mat comprising an electrical resistance element is powered electrically, the mat being electrically connected to two terminals of an electrical power supply and serving to heat at least a portion of the motor vehicle seat, the method comprising:
a) a first step in which the electrical resistance element is disconnected from at least one of the two terminals of the electrical power supply;
b) a second step in which a capacitive sensor of a measurement mat having at least one sensor is charged by being connected between a charging potential and a reference potential;
c) a third step in which the charge stored in the sensor during the preceding step is measured by disconnecting the sensor from the charging potential and connecting it to a measurement circuit for measuring electric charge; and
d) a fourth step in which the electrical resistance element is reconnected to the terminal from which it was disconnected in the first step so as to enable at least a portion of the seat to be heated.
In preferred implementations of the invention, use may optionally also be made of one or more of the following dispositions:
the second and third steps are repeated for each of the sensors of the measurement mat prior to moving onto to the fourth step;
during the first step, both terminals of the electrical power supply are disconnected from the electrical resistance element; and
during the second step, at least one of the two terminals of the electrical power supply is taken to the charging potential.